This invention relates to an electrostatic beam deflection scanner and beam deflection scanning method for use in a beam processing system using an ion beam or a charged particle beam. This invention is applied, for example, to an ion implantation system of the type that implants ions into a wafer by performing parallel scanning to the wafer with a high-current ion beam having low to intermediate energy. Particularly, this invention relates to improving the ion implantation uniformity in a scan direction by suppressing a zero electric field effect in an electrostatic beam defection scanner.
Referring to FIGS. 1A to 1D, a description will be given of the structure and operation of an electrostatic beam deflection scanner (hereinafter abbreviated as a “deflection scanner”) applied to an ion implantation system,
As shown in FIG. 1A, the deflection scanner has a pair of scanning electrodes 51A and 51B installed so as to be opposed to each other with an ion beam passing region interposed therebetween. Electron suppression electrodes 52 and 53 are installed on the upstream side (front side) and the downstream side (rear side) of the scanning electrodes 51A and 51B with respect to the beam traveling direction, respectively. The electron suppression electrodes 52 and 53 each have an opening in the ion beam passing region. A ground electrode 54 is installed adjacent to the downstream-side electron suppression electrode 53. As will be described later, the scanning electrodes 51A and 51B have mutually opposed electrode surfaces each having a circular-arc cross-sectional shape, Further, the interval between the mutually opposed electrode surfaces gradually increases as going from the upstream side to the downstream side.
An ion beam entering this type of deflection scanner is composed of cations of a required ion species and electrons attracted to these cations are attached to the ion beam.
The deflection scanner is electrically connected to a non-illustrated AC power supply and an AC voltage from the AC power supply is applied across the scanning electrodes 51A and 51B. Voltage application to the scanning electrodes 51A and 51B of the deflection scanner is roughly classified into the following three states.
1. As shown in FIG. 1B, a negative voltage is applied to the scanning electrode 51A, while a positive voltage is applied to the scanning electrode 51B.
2. As shown in FIG. 1C, a positive voltage is applied to the scanning electrode 51A, while a negative voltage is applied to the scanning electrode 51B.
3. As shown in FIG. 1D, the voltage is zero at both the scanning electrodes 51A and 51B. This represents a time when the positive and negative potentials applied to the scanning electrodes 51A and 51B are switched. In other words, by switching the positive and negative potentials applied to the scanning electrodes 51A and 51B at a constant period, an ion beam reciprocatingly scans a predetermined scan range at the constant period. This scan is called a parallel scan with electrostatic deflection using varying electric field.
In FIG. 1B, ions with positive charge passing through the deflection scanner are attracted to the left-side scanning electrode 51A having the negative voltage. On the other hand, electrons attached to an ion beam are attracted to the right-side scanning electrode 51B having the positive voltage. While passing through the deflection scanner, the ion beam loses the electrons and thus the ions with positive charge repulse each other due to the space-charge effect, so that the ion beam tends to diverge. Since the mass of an electron is smaller than that of an ion, the deflection angle of the electron is greater than that of the ion.
Also in FIG. 1C, for the same reason as in FIG. 1B, an ion beam tends to diverge while passing through the deflection scanner.
On the other hand, in FIG. 1D, since the voltage is zero at both the scanning electrodes 51A and 51B of the deflection scanner, an ion beam passes straight between the scanning electrodes 51A and 51B. Since electrons attracted to the ion beam also pass straight without being attracted to the scanning electrodes 51A and 51B, the ion beam passing through the deflection scanner tends to converge to some extent by the remaining electrons. This phenomenon is often called a zero electric field effect. This type of deflection scanner is disclosed, for example, in a document 1 (Japanese Unexamined Patent Application Publication (Tokuhyo) No. 2003-513419).
When the ion beam scans the scan range with the electrostatic deflection using the varying electric field as described above, there arises a problem that, as shown in FIG. 2A, the beam diameter becomes larger at end portions of the scan range than that at the central portion of the scan range on the downstream side of the deflection scanner. The beam diameter represents a cross-sectional size of the ion beam in a scan direction (indicated by arrows in FIG. 2A) in the reciprocating scan plane of the ion beam. The end portions of the scan range represent end portions near the scanning electrodes 51A and 51B in the reciprocating scan plane of the ion beam, while the central portion of the scan range represents a portion around the central axis in the reciprocating scan plane of the ion beam.
The above problem arises because the mass of each ion contained in the ion beam and that of each electron caught in the ion beam largely differ from each other and further because the repulsion force between the ions increases as the beam current density increases.
If the beam diameter of the ion beam changes in the process of the parallel scan with the electrostatic deflection as described above, the density of the ion beam increases when the ion beam passes through the scan range, so that the amount of ion implantation into a wafer increases, while, the density of the ion beam passing through the end portions of the scan range decreases due to divergence. As a result, the ion implantation uniformity in the wafer is degraded.
This problem commonly exists in ion implantation systems employing an electrostatic deflection scanner.
The Influence of such a problem is small in an ion implantation system using a high-energy ion beam or an ion implantation system using an intermediate-current ion beam. However, in an ion implantation system using a low-energy high-current ion beam, the ion implantation uniformity is degraded more significantly, which thus arises as a serious problem.